A planet is an object which orbits a star.

Planets must have three things to be classed as planets and not other objects, such as dwarf planets, these are:

• Be spherical in shape, rounded by its own gravity.
• Be small enough not to becomes stars themselves, by carrying out nuclear fusion.
• Have cleared it's local area of other objects, such as asteroids.

Planets tend to be divided into terrestrial planets, with a solid surface, and gas giants, which are made of layers of gas with small rocky cores.

The terrestrial planets in our Solar System are:

• Mercury
• Venus
• Earth
• Mars

The gas giants in our Solar System are:

• Jupiter
• Saturn
• Uranus
• Neptune

Planets which orbit other stars (i.e. NOT the Sun) are called exoplanets.

Facts and Figures	Mercury	Venus	Earth	Mars	Jupiter	Saturn	Uranus	Neptune
a 1 AU (or Astronomical Unit) = 149,600,000 km and is the mean distance from the Earth to the Sun b 1 Earth Mass = 5,980,000,000,000,000,000,000,000 kg c 1 g = 9.8 m/s2
Orbital distance (AU) a	0.38	0.72	1.00	1.52	5.20	9.45	19.20	30.06
Radius (km)	2,440	6,054	6,378	3,397	71,492	60,268	25,559	24,746
Mass (Earth Masses) b	0.055	0.82	1.00	0.11	318	95.2	14.5	17.1
Year Length (Earth days)	88	225	365.25	687	11.9 years	29.45 years	84.0 years	164.8 years
Day Length (Earth Days)	176	117	1.0	1.03	0.41	0.43	0.75	0.67
Surface Gravity (g) c	0.38	0.91	1.0	0.38	2.34	0.93	0.92	1.12
Surface Temperature (approx.)	-200 to 400°C	460°C	-80 to 50°C	-150 to 20°C	-110°C	-140°C	-190°C	-200°C
Number of Moons	0	0	1	2	79	82	27	14